Definition
The rate at which the elevation of the ground rises or falls over a given horizontal distance, expressed as a slope or grade beneath the aircraft's flight path.
Plain English
How steeply the ground is climbing or dropping under you as you fly along.
Context Anchor
Seen in ground-proximity warning system discussions, especially when flying near rising ground, mountains, valleys, or uneven terrain.
Derivation
From Latin terra (earth, ground) and gradus (step, slope). A 'gradient' literally describes the steepness of a step or slope, so 'terrain gradient' is simply the slope of the ground itself.
Why Pilots Care
GPWS systems factor in terrain gradient to distinguish between gradual slopes and rapidly rising ground that could lead to controlled flight into terrain.
Analogy
Think of walking toward a hill. On flat ground, your height above the ground stays about the same, but as the hill rises toward you, the ground gets closer to your level with each step.
Grounding Statement
If the ground ahead rises faster than the aircraft is climbing, the aircraft’s clearance above the ground is shrinking.
Intuition Check
Terrain gradient does not mean the aircraft’s climb gradient. It means the slope of the ground itself—the rate at which the terrain rises or falls over distance.
Example Sentence 1
The GPWS issued a terrain warning because the rising terrain gradient ahead exceeded the system's safe clearance threshold.
Example Sentence 2
Pilots review terrain gradient data when planning departures from airports surrounded by rising slopes.